1. FIELD OF THE INVENTION
This invention relates to a multi-color film electroluminescent display apparatus of a matrix type in which apparatus a plurality of EL luminous layers are provided on a plane, and a production process thereof.
2. DESCRIPTION OF THE RELATED ART
A display apparatus constructed of a plurality of electroluminescent (hereinafter referred to as EL) elements having different luminous colors is described as an example of first conventional technique in Proceedings of the 3rd International Display Research Conference, p 570 (1983), Kobe. The structure of the conventional apparatus is illustrated in FIG. 4. This apparatus comprises first illuminant layer 33 comprising ZnS:SmF.sub.3 (0.5% by weight) interposed between dielectric films 32 and 34 such as Y.sub.2 O.sub.3 or the like; said illuminant is provided on a glass base 30 through first transparent electrode 31, and second illuminant layer 37 comprising ZnS:TbF.sub.3 (2% by weight) interposed between dielectric films 36 and 38 such as Y.sub.2 O.sub.3 or the like; said illuminant is provided on the first illuminant layer 33 through second transparent electrode 35 such as ITO or the like. It is possible to obtain red luminescence having a wave length of about 650 nm from the first illuminant layer 33 by impressing an alternating voltage between the first electrode 31 and the second electrode 35 and green luminescence having a wave length of about 540 nm from the second illuminant layer 37 by impressing an alternating voltage between the second electrode 35 and an uppermost electrode 39.
On the other hand, display apparatuses comprising a plurality of EL elements on a plane and production processes thereof are described, for example, in Japanese Patent Application Kokai (Laid-Open) No. 257097/85. In this conventional technique, a transparent electrode having a thickness of about 0.1 82 m is formed on a transparent substrate, and a first dielectric film comprising Y.sub.2 O.sub.3 and having a thickness of about 0.4 .mu.m is formed by electron beam deposition or sputtering methods. Next, a resist film having a thickness of 2 .mu.m is formed on the dielectric film, and a light absorber having a thickness of 0.4 .mu.m is then formed in a shape of a lattice by a resistance heating method or the like. A protective layer comprising, for example, SiO.sub.2 is formed on the aforementioned light absorber. After ultrasonic cleaning of the aforementioned substrate by dipping into an organic solvent such as acetone, the resist film and the light absorber and the protective film thereof on the resist film are removed completely, and a resist film having a thickness of about 2 .mu.m is formed again into a predetermined shape. Then, the first illuminant layer, for example, a ZnS film containing 2% by weight of TbF.sub.3 and having a thickness of about 0.4 .mu.m is formed by an electron beam deposition method, an ion beam sputtering method or a magnetron sputtering method. Then, the substrate is subjected to ultrasonic cleaning in an organic solvent such as acetone to remove the resist film. In order to remove completely the resist film, an etching mask is formed with the use of, for example, a resist film so that an aperture with a predetermined shape is provided only on the first illuminant on the resist film. Then, the first illuminant on the resist film is removed by an ion beam etching method with the use of argon gas or the like to expose the resist film, and then ultrasonic cleaning is carried out in an acetone organic solvent. Next, a resist film is formed again in the same manner, and the second illuminant, for example, a ZnS film containing 0.5% by weight of SmF.sub.3 is formed by an ion beam sputtering method.
When a display apparatus comprises three luminant colors, it is necessary to add further an additional process which is similar to those described above. After the second or third illuminant layer is formed, a resist film is completely removed by the same process as above.
Then, after the back surface of the substrate is protected with a resist or a tape, the substrate is dipped into a mixed solution of hydrofluoric acid and ammonium fluoride (NH.sub.4 F) to remove completely a protective layer and an illuminant on the protective layer.
In the above-mentioned first conventional technique, the second illuminant layer 37 is formed on the uppermost layer wherein several layers of film are superposed, so that it is significantly affected by the factors such as unevenness of the base substance, stain or the like during the course of the production of a film, and the film thus formed is deteriorated extensively as compared with a film of the illuminant layer 33 simply formed on the dielectric layer. Thus, it is known that brightness from the aforementioned illuminant layer comprising two layers is decreased in a range of 60-70% as compared with that from a single layer. Further, as for the electrode for operating the first illuminant layer 33, both of the upper and lower electrodes are ITO films 31 and 35. It is also well-known that such an electrode construction as above tends to spread its dielectric breakdown range of elements as compared with an electrode construction comprising an ITO film and an Al film. It is also considered that in such a construction as in the conventional example, defects of a film, localized deposition of impurities or stains per unit area or the like are necessarily in a higher level rather than in an element comprising single layer illuminant and thus the former construction tends to be broken down. On its breakdown, the first and second illuminants 33 and 37 become inoperable by the damage of the second electrode 35. Thus, when a matrix type EL display apparatus is constructed with the EL elements having the aforementioned construction, a problem is encountered in that the number of the defects of the picture elements increases as compared with the case of the illuminant comprising a single layer.
Next, in the aforementioned second conventional technique, a resist film is directly formed on an Y.sub.2 O.sub.3 dielectric film or an illuminant film, so that the interfaces of dielectric films or illuminant films which effect seriously the properties of the EL element are contaminated by impurities. Furthermore, ultrasonic cleaning in an organic solvent such as acetone and then dipping into an oxygen plasma atmosphere for removing the aforementioned resist film enhances the contamination and possibly deteriorates the properties of the dielectric film itself and the crystal properties of the illuminant film. All of these deteriorative effects will apparently cause the deterioration of the EL element and thus cannot be accepted as such. Further, in the second conventional technique, a complicated process comprising at least 15 steps is required even for producing an EL display apparatus comprising two color illuminant films. The process exceeds 20 steps by the addition of essential steps such as drying, thermal treatment and the like, and thus a serious problem which cannot be accepted from the industrial viewpoint of cost or yield, was encountered in the second conventional technique.